roach (n.1)

shortened form of cockroach, 1837, on mistaken notion that it was a compound. In contemporary writing said to be from a polite desire to avoid the sexual connotation in the first syllable. Meaning "butt of a marijuana cigarette" is first recorded 1938, perhaps from resemblance to the insect, but perhaps a different word entirely.

roach (n.2)

small freshwater fish, c. 1200, from Old French roche (13c.), of uncertain origin, perhaps from a Germanic source. Applied to similar-looking fish in North America.